Many local area network (LAN) products use a medium formed by twisted copper wire pairs for the transmission and reception of data. For these products, there is typically a requirement to use one or more pairs for the transmission of data, and one or more pairs to receive the data. For LAN technologies which employ a full-duplex medium, many have different pairs for transmission and for reception and thereby require that each end of the link use a defined assignment for its twisted pair connector to a particular pair of wires. For example, a Network Interface Card (NIC) is usually employed as an end node device, while a switch or repeater will be employed as a concentrator or central component in a star-based network. In order for the link to operate, the NIC must transmit on a pair of wires that is connected to the receiver of the hub or switch and the NIC's receiver must be connected to the transmitter at the other end of the link. If the transmit pair of wires of a device is connected to the transmit connectors of the connected device, the communication link will typically fail.
Most LAN standards address this by assigning different connector pins to the wires in the twisted pair cable which is typically referred to as the Media Dependent Interface (MDI). In the IEEE-802.3's 10BASE-T standard, an end node will assign pins 1 and 2 to the transmit pair, while pins 3 and 6 are for connection to a twisted pair for receiving. The repeater for a 10BASE-T network will assign its transmitter to pins 3 and 6, while its receiver will be connected to pins 1 and 2. This works very well for general configurations where NICs are attached to repeaters. However, there are a few cases that become more important to consider as the emergence of switched networks takes place. First, there is a case where the NIC is connected directly to another NIC, or a repeater is connected to repeater, or a repeater is connected to a switch. Depending on the assignment of pins at the product's network interface, it may become necessary to employ a “crossover” cable to address the fact that both products employ the same pin designations on their interfaces.
Some products will use a manual “MDIX” (Media Dependent Interface Crossover) switch that allows them to connect to other similar devices but requires an installer to manually push a button. This approach works well for very limited applications like a repeater-to-repeater port but it would not work well in the general case where it is desired to build a product that attaches to either repeater, NIC, or switch without manual intervention. To overcome this drawback, a new approach must be used.
In a prior product known as the 100VG Modular Transceiver from Hewlett-Packard Company, bus switches were employed to make a solid-state version of this type of circuit. However, that prior product used a very low frequency alternating pin selection to perform its “automatic” operation. While the algorithm in the product was suitable for its intended purpose, it had the drawback that if two such devices were attached together, they could potentially get into a “lock-step”operation where the necessary connection for establishing a communication link became impossible. Subsequently, other products came out which employed a similar algorithm and were forced to use a different alternation rate to avoid the lock-step phenomena. While that approach was suitable for its intended purpose and worked for a very limited set of products, it did not solve the general case whereby a guaranteed connection between nodes was achieved regardless of the manufacturer or the model of the product that was being used. Therefore, to guarantee (or at least assure a very high probability of) connection, another method is needed to avoid the “lock-step” interaction between automatic crossover circuits.